Romaine lettuce should provide a product at harvestable stage, which is accepted by processing industry and/or consumers. Therefore the harvestable product should not have tipburn, should have a short core, and it should have thick, strongly blistered leaves with a dark green outer leaf color. Further it should be sufficiently headed, and as a result of this heading provide sufficient yellow-colored heartleaves.
Mature romaine lettuce is mainly harvested by hand. This harvesting process is labor-intensive, and relatively laborer-unfriendly. As it getting more and more costly to hire laborers that want to work outdoor in the field, close to the ground and under all kinds of adverse weather conditions, there is strong interest of the lettuce industry to mechanize this manual harvesting process. However, until now the lack of uniform quality among mature plants is a major problem for mechanical harvest. Poor quality of basal leaves, internal breakdown of heartleaves due to tipburn, fringe burn of leaf margins and fast bolting are the main quality problems. If one would like to mechanize the harvest process, these quality problems result in additional hand sorting, which is costly, and/or a very low net yield, if the plant parts with a high risk of quality problems are mechanically discarded. The latter approach is chosen by some producers that produce romaine hearts, i.e. the intact yellow-green heart of the romaine lettuce without the green outer leaves. For producers that produce romaine lettuce for processing, i.e. pre-washed and pre-cut leaves, this is not feasible because of four major constraints.
The first is the requirement for a mix of green and yellow leaves, which requires the preservation of the undamaged green outer leaves. The second constraint is the strongly reduced net yield, which is a result of the fact that an automated knife should cut through the leaf but not through the core. Especially fast bolting lettuce varieties are unacceptable for mechanized harvest of romaine lettuce for processing purposes because of their long core.
The third constraint is caused by the use of mechanically-driven horizontal knives that easily damage the hollow main veins of romaine leaves and cut halfway through the prostrate, round-shaped, lower outer leaves, which results in wide cut surfaces. Cutting damage on a leaf with a hollow main vein is often not restricted to the cut surface but bruising extends into the leaf along the main vein.
The fourth constraint is the requirement for a leaf to have a good shelf life. This means the leaves should get through the washing and drying process in the factory without bruising or breaking. The leaves should be sufficiently thick and tough to avoid wilting. In addition, the leaves should be strongly blistered to avoid sticking to each other and to the plastic of the packaging, which increases local humidity on the leaf and reduces local respiration.
There exists a need, therefore, for a new type of romaine or cos lettuce, which is suitable for mechanical harvest with a simple horizontal knife and which exhibits resistance against downy mildew (Bremia lactucae). The harvested product is meant for processing purposes.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.